An alkyl glycoside, which is a sugar derivative surfactant which is less irritating than other surfactants. Also, though it is a nonionic surfactant, alkyl glycosides form stable foam per se, and, furthermore, exert a foam-stabilizing effect on other anionic surfactants. These characteristics have made alkyl glycosides highly noteworthy.
Although alkyl glycosides as novel surfactants, have the above-mentioned noteworthy characteristics, it is quite difficult to produce them in the form of a commercially useful product.
An alkyl glycoside is produced by reacting a sugar with an alcohol. During this process, however, it is a most serious problem that various procedures in the process frequently cause deterioration of the hue.
When an alkyl glycoside is to be produced by reacting a higher alcohol with a sugar, in particular, the higher alcohol is generally used in large excess with respect to the sugar. This fact indicates that it is very important to recover and reuse the unreacted higher alcohol from the reaction system in order to more economically produce the alkyl glycoside on an industrial scale.
When the unreacted alcohol is recovered by, for example, distillation from the reacted mixture, which will be called the "recovered alcohol" hereinafter, and is to be used as such in the subsequent production of the alkyl glycoside, however, there is a serious problem that the alkyl glycoside thus formed shows a considerably deteriorated hue.
Several methods have been reported for inhibiting the deterioration of hue in the production of an alkyl glycoside. In the production of an alkyl glycoside through a reaction of a higher alcohol with a monosaccharide, for example, JP-A-59-139397 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a process wherein said reaction is effected in the presence of an acid catalyst composition comprising an acid catalyst and a reducing agent; European Patent 0132043 discloses a process wherein an acid form of an anionic surfactant is used as a catalyst; and European Patent 0132046 discloses a method wherein the neutralization with an organic base is conducted at the termination of reaction. In the step where the obtained alkyl glycoside is separated from the unreacted and recovered alcohol, furthermore, JP-A-62-192396 discloses a process wherein a viscosity depressant is added, since the high viscosity and poor heat stability of the alkyl glycoside would cause particularly serious deterioration of the hue.
However none of these methods can provide an alkyl glycoside which shows a satisfactory hue when applied to a commercial product in practice. JP-A-61-33193 (corresponding to U.S. Pat. No. 4,557,729) proposes to bleach the finally obtained alkyl glycoside with hydrogen peroxide and sulfur dioxide. However this is not a rather drastic measure, and is accompanied by other problems, such as deterioration with respect to odor and a poor stability upon storage.
The above-mentioned facts clearly indicate that it is necessary to regenerate the recovered alcohol to thereby make its qualities comparable to the unused one with regard to, at least, the causes for the coloration, when the recovered alcohol is to be reused in the production of a fresh alkyl glycoside.